Soil contamination by radioactive material has occurred widely as a result of mining operations, nuclear power plant operation, nuclear device testing, and the like. Radioactive species commonly found in contaminated soils, ores, and other particulate matter include radium, uranium, and thorium. For example, soil contamination by heavy metal billets, such as those made of uranium, which have been used as a high density material in military or civil applications, is an important soil contamination problem.
Existing technologies for treating contaminated radioactive soils include underground storage, incineration, and soil washing. Soil washing uses both physical and chemical extraction to remove wastes, and results in a reduced volume of contaminated soil to be disposed. Many soil washing methods require large volumes of water, and are thus impractical in areas or settings where fresh water supplies are limited. In addition, the water used in the method may itself need to be decontaminated before it can be safely stored.
Conventional uranium mining technology may not be applicable to many soil remediation applications. Drawbacks of conventional uranium mining technology, when applied to remediation efforts, include the requirement of fresh process water of about 1-1.5 tons per ton of ore processed. Fine grinding to pass 28 mesh (590 micron) screens is required for most ores due to the tight inclusion of uranium minerals in the rock matrix due to tens of millions of years of geologic formation. Fine grinding generates large quantities of fines which are difficult to process. Additionally, tailing ponds are required for impoundment of wastewater generated in the process. Safety concerns surrounding the possibility of liner breakage and water seepage from the standing water remaining in the ponds, especially in certain climatic, seismic, and densely populated regions, make tailing ponds an undesirable alternative.
Reagent selection for traditional uranium mining operations is based on effectiveness and costs of chemicals in relation to yield rather than on the quality and impact of the waste generated by the process. The final stages of uranium extraction processes usually employ the extraction of uranium salts such as uranyl sulfate or uranyl nitrate from aqueous solutions by utilizing large volumes of organic solvents. Many of these solvents post health hazards for workers, and all are environmentally undesirable since they generate mixed wastes.
In contrast, the goals and objectives of a program for removing depleted uranium from contaminated soils for remediation purposes mandate a much different approach which overcomes the limitations of traditional uranium ore extraction procedures and overcomes the drawbacks of existing radioactive-material remediation techniques.